The present invention relates to a method and a device for treatment of a gas. The invention is particularly applicable for energy recovery and/or removal of CO2 from exhaust or flue gas.
When using gas turbines for converting thermal energy into mechanical energy, it is common to produce steam from the waste heat and use this steam to drive a steam turbine, thus increasing the overall energy efficiency. Adding steam production and a steam turbine to a gas turbine energy conversion process today involves very bulky process equipment. The boiler in particular is voluminous. The piping is also very substantial and complex. In an offshore environment as on an oil/gas platform it is therefore normal practice to install the gas turbine without any device for energy recovery from the exhaust gas since gas is cheap in such an environment while the space is at a minimum.
With the increasing focus on carbon induced global warming and introduction of carbon dioxide or energy taxes, this situation is being revaluated. Increasing the energy efficiency of the process means that less climate gases are emitted. There is also focus on the possibility of sequestering carbon dioxide from the flue gases from such energy conversion plants.
Carbon dioxide recovery demands that the temperature of the exhaust gas is reduced to lower levels than the typical gas turbine exhaust gas temperature of 500xc2x0 C. Highest temperature allowable for separating carbon dioxide from gas with today""s technology is in the region of 100-150xc2x0 C., but more typical for exhaust gas treatment would be 20-50xc2x0 C.
When treating the exhaust gas with liquid based methods (e.g. absorption), droplets will usually become present in the gas phase. Droplets may also arise when condensation occurs. Thus it is desirable to remove such droplets either to recover the liquid or to avoid an extra effluent to the atmosphere.
A conventional design for a plant to achieve heat recovery, cooling, and carbon dioxide removal from exhaust gas will embrace a separate boiler, followed by a cooler for the gas before the carbon dioxide is removed in an absorption column with a demister unit downstream, and probably a blower to overcome the ensuing pressure drop. These process units are bulky and require requiring instrumentation and control, in addition to a complex piping system to connect the various units.
It is known from the literature (e.g. Sawyer""s Gas Turbine Engineering Handbook, 3rd ed, vol 11, chapters 7 and 14) how the energy recovery problems in gas turbine cycles may be solved technically, but the known solutions are costly in general, and too bulky for applying offshore. Furthermore, these technical solutions are associated with finite pressure drops that will lead to higher gas turbine exit pressure with reduced energy efficiency as a result. Alternatively some kind of blower can be installed to overcome the pressure drop.
It is further known from the literature (e.g. W. W. Bathie, Fundamentals of Gas Turbines, 2nd ed., Wiley, 1996, chapter 8) that gas turbine blades are cooled by internal water flow. The technique as it stands is designed to keep the blade surfaces cool enough to avoid material failure, and is not suitable for efficient heat transfer or energy recovery.
Also known from the literature (e.g. Kohl and Nielsen, Gas Purification, 5th ed., Gulf Publishing, 1997) is how carbon dioxide may be removed from gas by various means. Removal of carbon dioxide from flue gas represents a special problem due to lack of available pressure and the presence of contaminants like nitrogen oxides and oxygen in the gas. One such application is described in the literature (Pauley et al., Oil and Gas Journal, May 14, 1984, pp 84-92). Many developments to improve such technology have in recent years been invested in (see e.g. Greenhouse Gas Control Technologies, edited by Eliasson, Riemer and Wokaun, Pergamon, 1999).
GB Patent No.1,332,684 teaches how heat exchangers may be attached to the rotating assembly of an air compressor combined with combustion chamber and gas turbine. It is shown how heat from the gas turbine exhaust may be used to preheat combustion air via an intermediate heat transfer medium operating in a closed loop including heat exchanger matrices placed in the exhaust gas stream and the air, respectively. The disadvantages of this invention include the creation of a pressure drop in the exhaust gas through using said gas to hydraulically drive the heat exchange assembly, and a limited potential for heat recovery. The physical placement of the heat transfer facility and the created pressure drop will also interfere with the design of the gas turbine assembly. Furthermore, the heat exchanger can not be mounted as a retrofit.
The patents DE 33 26 992, EP 262 295, and EP 556 568 deal with various features of what is essentially the same invention or developments thereof. They teach how an apparatus may be placed in the exhaust duct from any combustion engine, and how that apparatus can be formed to recover heat and kinetic energy from the exhaust gas. Furthermore, they teach how steam may be produced to feed a steam turbine, and how this apparatus may be designed to sit on one rotating axle from which power is transferred to the combustion engine""s crankshaft. This invention is intended to increase the efficiency or power output from a combustion engine. The pressure drop induced in the exhaust gas stream is a disadvantage, but less so for a combustion engine than a gas turbine. The narrow flow path of the exhaust gas with or without turbine blades leads to considerable pressure drop. A further disadvantage is that steam is formed in a single chamber such that only one temperature level is possible, thus effectively configuring the heat transfer as cross-flow with the limitation this gives with respect to heat recovery. It is yet a further disadvantage that the temperature of the exhaust gas can not be reduced to a level where carbon dioxide removal can be performed.
WO 98/30486 describes how a compact heat pump may be constructed by combining transport facilitating features like compression and pumping on the same axle. This patent application also describes a heat exchanger in the shape of a spiral wound in a tight annular space where one medium flows inside the spiral tube and the other medium flows in the annular space. This solution will have difficulty in coping with the large gas flows normally associated with handling exhaust gas or large gas based processes.
U.S. Pat. No. 5,363,909 teaches how a gas can be piped into a chamber in which a rotating packing is placed, and where the gas is forced to flow radially towards the center of said packing while being in contact with a liquid moving radially outwards. Cooling or heating can take place by direct heat transfer, but the direct contact would make it impossible to produce useful steam or any other useful heat medium. And it represents separate equipment and a significant pressure drop since the said packing will impart some fan action on the gas trying to force it outwards, and this action must be overcome by extra pressure drop.
SU 1189473 teaches how a gas flow through a column in which concentric discs sloping towards the axis are fixed on the inside wall, and in the center is placed a rotating entity with discs sloping outwards. Liquid flows over the rotating discs and is flung to the wall from where it flows over the static discs back on to the rotating disc below. The apparatus is intended for mass transfer, but by letting the liquid evaporate, cooling of the gas may be effected. Again it is impossible to make any use.
SU 359040 teaches how a gas and a liquid can be contacted by letting the gas flow through a horizontal tank at the bottom of which there is a liquid pool. Liquid is splashed into the gas phase by rotating entities dipping into the liquid and flinging it out in the gas. And the rotating entities are driven by the flow of the gas. Mass transfer may be effected, as above, but the device is of little use in recovering heat from the gas. The mass transfer operation is no very efficient, e.g. with respect to space (m2/m3).
Further from the literature (e.g. A. Bxc3xcrkholz, Droplet Separation, VCH, 1989) there are known several methods for removing droplets and/or mist from gases.
Hitherto the technology for achieving heat and mass transfer for large flows of gas has been bulky and costly. If this problem is to be effectively solved, new technology will be needed.
The main object of the present invention is to provide a compact and multifunctional device for treatment of gas.
The inventor has developed a method and a device for treatment of exhaust gas or flue gas which is a significant improvement upon today""s available technology. The device is compact and multifunctional, and it reduces the need for piping by building the device into the exhaust gas duct where heat and mass transfer will be achieved. Optionally some axial movement may be imparted on the gas to avoid pressure drop in the duct, even a negative pressure drop could be foreseen.
The device according to the present invention comprises one or more entities 3 embracing functional elements 10,20,110,200,230,310,320,330,400 attached to, and which are able to rotate with, a hollow axle 4, and one or more pipes 5 mounted axially inside the hollow axle for transportation of necessary auxiliary fluids to effect the gas treatment.
Furthermore, the method according to the present invention comprises that the gas stream is brought in contact with one or more entities embracing functional elements attached to, and rotating with, a hollow, rotating axle and where necessary auxiliary fluids effecting the gas treatment flow through the functional element in each entity facilitated by pipes mounted axially inside the axle.
The invention reduces the complexity of the gas processing intended by enabling installation of all relevant processing and/or contacting devices in the gas duct itself, thus reducing the need for piping.
The invention comprises one or more entities embracing functional elements mounted on, and able to rotate with, a hollow, rotating axle in the exhaust gas duct, typically from a gas turbine, but the exhaust gas could equally well come from another source. The elements are constructed to combine functions like heat transfer, mass transfer and demisting. The functional elements are also shaped to aid the axial movement of the gas by providing some fan action. The axial movement aid included is basically there to ensure that the device causes no pressure drop by its presence in the exhaust gas duct. The rotation of the entity increases the turbulence around the functional elements, thus increasing the transfer of mass and heat as applicable. Stators may be mounted on the duct wall to improve the axial movement aid efficiency. The axle is mounted in the channel using suitable bearings that are cooled as necessary. The axis of the rotating entity is mounted in parallel to the axis of the gas duct. Said axes may coincide, but not necessarily. Some form of driver is also provided.
The invention is particularly applicable for energy recovery and/or removal of CO2 from exhaust gas or flue gas.
Hot gas in the exhaust gas duct will first be cooled to make steam or superheated steam by flowing through a rotating heat transfer element mounted in the duct as described above. The next step is further cooling of the gas by possibly raising more steam of a lower pressure, and then finally to cool the gas as required. The heat transfer elements needed may be mounted in series on the same axle, or they may be mounted on axles mounted in succession, but they are all intended to be placed in the duct.
The water, as liquid or steam, flows in ducts mounted internally in the axle which is hollow. The water will typically first be heated against the gas leaving, then the heated water will flow back inside the axle and proceed to the next hotter heat transfer element. The water flow through these elements will be partially in parallel and partially in series as beneficial according to heat transfer kinetics, heat recovery, and fluid flow considerations.
Once the gas is properly tempered, it will be lead through one or more entities, partially in parallel and partially in series with respect to the liquid absorbent used, where carbon dioxide in the exhaust gas is absorbed. The liquid absorbent is introduced in the core through the axle and makes its way to the periphery of the gas duct via an entity, or entities, enhancing the mass transfer surface. At the periphery the liquid is collected, drained and processed for recycling.
Downstream of the processing another entity is mounted on the axle, and this entity will act as a demister where liquid droplets entrained in the gas are collected and drained by centrifugal force towards the periphery of the gas duct from where it is collected and drained.